The present invention relates to a "reach-in" merchandiser type of open front refrigerated display case of cabinet used primarily in retail food and supermarket outlets.
More particularly, it relates to a refrigerated display cabinet having an air defrost system incorporated therein in which transparent barrier doors are retrofitted onto the cabinet in a manner which does not interfer with the air defrost system and enhances the energy efficiency of the cabinet.
The term "refrigerated", in accordance with the present invention is intended to incorporate those cases maintained at a temperature at or in excess of 32.degree. F., such as display cases utilized for the display of milk and fresh foods, and those cases maintained below 32.degree. F., such as frozen food cases. In addition, references are made herein to the use of transparent doors with such open front cabinets, since those are the type of doors most frequently utilized in such retail outlets. Other types of doors could also be employed within the scope of the present invention.
In the operation of all types of refrigerated display cabinets, it is desirable to include a system for automatically defrosting the refrigeration coils. The defrost cycle can be actuated either at set periodic time intervals or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. In this manner of operation it is possible to avoid any significant frost buildup within the display cabinet such that inoperability and spoilage of food products would occur.
There have been three different approaches for defrosting refrigerated display cabinets in this art. These are, utilizing electric resistance heaters; passing a compressed refrigerant gas having a high specific heat through the refrigeration coils; and, circulating ambient air through an air conduit in which the refrigeration coils are positioned. Due to the increased cost of energy, efforts have been made to place more emphasis on the utilization of ambient air defrost systems as an alternative to the electrical resistant heaters or compressed refrigerant gas defrost systems.
One type of system which employs ambient air during a defrost cycle is exemplified by U.S. Pat. Nos. 3,403,525; 3,850,003 and 3,937,033, all to Beckwith et al. These systems use fans separate and distinct from the main circulating fans to move ambient air across the refrigeration coils for defrosting. The additional fans are turned on only during the defrost cycle of operation for pulling ambient air from outside of the display cabinet directly into the air conduits located within the walls of the cabinet. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display cabinet. Such ports are normally closed during the refrigeration cycle and opened during the defrost cycle. The Beckwith et al U.S. Pat. No. 3,850,003 indicates that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commercially feasible.
Another type of ambient air defrost system is shown in U.S. Pat. No. 4,144,720 issued to Subera et al which is assigned to the same assignee as the present application. In the Subera patent, an open-front refrigerated display case having primary and secondary air conduits is disclosed. In this system the direction of the air flow within one of the conduits is reversed, for example, by the use of reversible fans to provide ambient air defrost. U.S. Pat. No. 4,026,121 to Aokage et al, which illustrates an open-front display case, and U.S. Pat. No. 4,120,174 to Johnston, which illustrates an open-top display case, also disclose reverse ambient air flows for defrosting.
In those ambient air defrost systems disclosed in the above-noted patents which use a reverse air flow, during the defrost cycle of operation, ambient air can easily be drawn through the access opening of the case or cabinet into the air conduit through the outlet opening located in the air conduit, and then expelled from the air conduit after the defrost operation through the inlet opening, and then forced out of the cabinet through the unblocked access opening. Such an arrangement, however, cannot be readily used in a refrigerated merchandiser display case having barrier doors, since the front opening in the cabinet is covered by the doors. Thus, in order to employ an ambient air defrost system, a different type of system had to be conceived.
In seeking to employ ambient air defrost techniques in cases having doors, systems have been developed for drawing air over a limited portion of the air conduit by opening flaps into the conduit, which flaps are arranged so as to straddle the evaporator coils of the refrigeration mechanism. Such systems are disclosed in U.S. Pat. No. 3,226,945 to Spencer and U.S. Pat. No. 4,072,488 to Johnston. The patent to Spencer illustrates a plurality of different embodiments of open-top refrigerated display cases, both of the single shelf and multi-shelf types, in which the air flow is always drawn over evaporator coils in a single direction under negative pressure. During the refrigeration cycle of operation, air after being refrigerated is circulated through the air conduit and into the display section of the case. The patent to Johnston discloses a glass door type merchandiser display cabinet in which air is circulated through the air conduit and through the evaporator coils arranged within the air conduit in such a direction that cold air enters the display space at the bottom of the cabinet and is then drawn up into the air inlet located near the top of the cabinet. For defrosting, top flaps are opened since this case is designed with coils at the top. This shows a somewhat complicated way to provide both glass doors and air defrost features according to the prior art. Such systems are relatively complex and can involve certain operational problems, particularly due to frost and dust accumulation. Where there are moving parts inside of the air conduit an accumulation of frost on such parts can cause them to stick and hence not function properly.
The prior art as represented by the patents discussed above has treated the opening of the barrier doors on such merchandiser refrigerated cabinets as being only a problem as illustrated by Johnston, U.S. Pat. No. 4,072,488 which describes the frost buildup due to the opening of the cabinet doors.
The prior art does not appear to have viewed the opening of the doors as a possible solution to the defrosting requirements.
The background of the invention described and claimed in the present application also includes a recognition of the energy conservation trend among managers of retail food outlets to reduce operating costs wherever possible. One such possibility for energy conservation is to provide heat transfer constraining barrier doors across the access opening of the refrigerated merchandiser display cabinet in order to reduce the contact between the ambient air which has high heat and moisture content and the refrigerated air within the display cabinet. Such barrier doors are often constructed of double or triple layer glass or other transparent materials, such as plastic sheets.
During periods of high door opening frequency for shopping or stocking the case or when the store ambient heat and humidity levels are elevated the refrigerated air band which may be at a temperature as low as -15.degree. F. is contacted by ambient air having a temperature as high as 75.degree. F. This contact can raise the refrigeration load even above that required by multi-air band open front cases having no barrier doors. To solve this problem it is optimum to employ one or two guard air bands which can protect the inner refrigerated band against direct contact with the ambient air when the merchandiser doors are opened.
Beckwith et al U.S. Pat. No. 3,403,525 also discloses a night curtain which is to be placed over the normally open access area of a refrigerated case in order to reduce energy consumption during the "non-sales" hours, but with this arrangement no air defrost or customer entry is possible.
Vogel, U.S. Pat. No. 4,117,698 discloses a retractable night curtain for use during closed store hours during which no provision is made for customer entry.